Many forms of power generation exploiting renewable energy sources employ electrical generators, and cause the generator to be driven at a speed that varies according to the prevailing conditions. For example, a wind turbine drives its generator at a speed dependent on the wind speed, a tidal-current turbine drives its generator at a speed dependent on the prevailing speed of the water flow, and a wave power converter may include a generator that is driven at a speed that varies in a cyclic fashion. In such cases, the voltage and frequency of the electrical output from the generator constantly vary. However, electrical grids operate at fixed voltage and frequency and strict regulations embodied in a grid code are applied to generating systems that are to be connected to the grid.
For large generating systems such as wind farms or groups of tidal-current turbines, grid codes typically include the following principal requirements:                1. The generating system should be able to contribute to the control of grid frequency by having the capacity to modify its real power output independently of the prevailing wind speed, tidal velocity etc.        2. The generating system should be able to contribute to the control of grid voltage by having the capacity to supply reactive power up to a certain defined proportion of the real power supplied.        3. The generating system should be able to produce current with a sinusoidal waveform. The current should not have a DC component, and all harmonic and sub-harmonic components should be less than a prescribed upper limit.        4. The generating system should be able to remain connected to the grid throughout a low-voltage transient fault on the grid, and so be ready to continue supplying power when the fault is cleared.        
In order to comply with the first of these regulations, most large wind turbines include a pitch control mechanism to adjust the mechanical power produced by the blades up to a maximum value corresponding to the prevailing wind speed.
To comply with the second and third requirements, the electrical power from the generator is converted to fixed-frequency, 50 or 60 Hz, three-phase ac by a power electronic system usually employing a pulse-width-modulated (PWM) inverter.
To comply with the last requirement, the inverter is controlled to restrict the output current rather than simply disconnect to protect the inverter from short circuits.
Suitable inverters are available with output voltage ratings up to 690Vrms line-line, which is one of the preferred standard voltages. Higher voltages are possible, but they are achieved at the expense of increased cost, lower efficiency, and lower modulation frequency, leading to higher harmonic currents sent to the grid and hence the need for auxiliary filters. It is normal therefore to use a 690V inverter connected to the grid via a transformer that steps up the voltage to the grid voltage at the point of connection.
For a land-based wind turbine this arrangement is satisfactory. However, for proposed tidal current turbines this arrangement presents several difficulties.
Firstly, a tidal-current turbine situated on the seabed would require a very large and expensive sub sea enclosure to house conversion equipment of the form described. Any fault in the equipment would require expensive marine operations to retrieve and replace the equipment, and work can be delayed by the prevailing weather conditions for long periods. In general, it is desirable therefore that as much equipment as possible be located at the receiving end of the cable that brings the power from the turbine to shore for connection to the grid, or to an offshore platform housing a special substation receiving power from a collection of such turbines.
Because of the transmission distance, it is preferable to use a moderately high transmission voltage, to reduce transmission losses. For example, to transmit 5 MW over a distance of 5 km via a two-core cable having 250 mm2 conductor cross section with a transmission loss of 4% requires an operating voltage of about 10 kV. Voltages of this magnitude can be generated by electrical generators, and so the possibility exists of a system that requires no electrical power conversion equipment at the turbine.
Because of the high cost of sub sea cable and its installation, it is desirable to have as many turbines as possible sharing a common cable. If the electrical output of each turbine is AC, then their outputs must be synchronised if they are to be connected together. However, if the turbine outputs are DC, then they may safely be connected in parallel, provided that they can operate at the same voltage.
Because of the high cost and the risk of failures of pitch-control mechanisms in a sub sea environment, it is preferred to use turbines with fixed-pitch blades. Because of the simplicity and efficiency, it is further preferred to use an electrical generator driven directly by the turbine without an intermediate gearbox, and to use permanent magnet excitation of the generator field. The AC output is easily and efficiently converted to DC by means of diodes, which may be embedded in the generator winding or the terminal box. The rotational speed of the turbine and generator and the associated output power then depend on the prevailing flow and on the voltage of the DC system into which the turbine-generator-diode feeds.
A preferred electrical arrangement for a group of tidal-current turbines is therefore as illustrated in FIG. 1, which shows a series of turbine generator machines 10 producing direct current output (via rectifying diodes 12). The generator machines 10 are connected in parallel by a set of short cables 14 and a longer cable 16 to a receiving station 18. Receiving station 18 may be on shore, or may be provided on an offshore platform where access is possible for servicing the equipment. The output of the receiving station 18 is connected to the AC power grid.
A known method for converting HVDC to AC power is through the use of a thyristor inverter circuit. A three-phase current source thyristor inverter is illustrated in FIG. 2, having an array of thyristors 100. During operation, a DC voltage is applied across terminal A. The phases of the signals controlling the firing angles of the thyristors 100 are staggered, such that a three-phase AC signal is provided at terminals X, Y, and Z. Thyristor inverters provide the advantages of a combination of efficiency and reliability, along with being relatively inexpensive.
While current source thyristor inverters are known to be used for the conversion of HVDC to AC power, current source thyristor inverters are in general not suitable for grid connection, as they generates large amplitude harmonic currents and draw reactive power from the grid. Furthermore, a current source thyristor inverter relies on the grid for the voltage to turn off the thyristors at the end of their conduction period, and so it fails to operate during a low voltage grid fault.
Michael Owen, “Homopolar Electro-mechanical Rotary Power Converter (HERPC), IEEE Melecon 2004, May 12-15, 2004, Dubrovnik, Croatia, discloses a power conversion system for converting high voltage DC power to AC grid supply power, the system comprising a line voltage input for receiving a high voltage DC power input, a converter module for converting said high voltage DC power input to AC power, a synchronous motor, said synchronous motor driven by the AC power provided by said converter module; and a synchronous generator operable to provide AC output power for connection to a grid supply, wherein said synchronous generator is driven by said synchronous motor.